1. Field of the Invention
This invention relates to adhesive coatings designed primarily for protecting, restoring, repairing, or modifying surfaces of objects made of polymeric materials. They are particularly useful in application over worn, cracked, discolored, or otherwise defective objects made of polyacrylic, polystyrene, and polycarbonate resins.
2. Brief Description of the Prior Art
Acrylic coatings, also frequently called sealers, varnishes, or glazes, are commonly used in many industries. The reason for their application includes protection, restoration, surface smoothing, improving surface characteristics, and enhancing aesthetics of objects made of plastics, metal, wood, ceramics, and paper, as well as human and animal tissues.
Such coatings found, among others, many diversified uses in the dental field as protective sealants over tooth enamel and dentin, for repairing and/or reconditioning of dentures and orthodontic plates, as glazes over composite restorations, for sealing margins in restored cavities, for enhancing bonding and/or adhesion between dental restorations or cements and the tooth structures, and for restoring and/or repairing worn or damaged dental prostheses.
The coating used for such applications may be divided into two basic groups which differ in the way the transfer from the liquid (monomer) state to the solid (polymer) state is accomplished.
The first group consists of two parts, blending of which causes the material to cure (harden). Such parts may be identical, substantially similar, or different in their basic chemical composition. One such part contains a polymerization activator (accelerator) and the other contains a polymerization initiator (usually, although incorrectly, called a catalyst). The most commonly used activators are comprised of tertiary aromatic amines, examples of which are N,N-dimethyl-p-toluidine or N,N-bis(2-hydroxyethylo)-p-toluidine. The most frequently used polymerization initiator is benzoyl peroxide.
The second group of coatings represent materials curable by exposure to electromagnetic energy. UV or visible light are the most convenient forms of such energy. For applications involving human bodies, particularly in dentistry, materials designed for cure with light sources operating in the visible range are preferred due to concern for biological safety and because they offer better curing characteristics and more convenient handling.
Light-cured materials represent a one-component system which offers advantages over its two-component counterpart because of its simplicity and excellent reproduction of properties of the cured material. Light-cured materials are also less sensitive to adverse storage conditions, offering a long shelf-life with no need for refrigeration.
For light-induced polymerization to occur in a clinically or commercially acceptable time frame (a few seconds to a maximum of a few minutes), the sealant must be formulated using highly reactive acrylic monomers, and polymerization activators/initiators must be present at sufficient concentrations. The polymerization-initiating systems most commonly recognized as effective and biologically safe include .alpha.,.beta.-diketones, representative of which are camphoroquinone and benzil, and various tertiary aliphatic amines, for example, diethylo (or dimethylo) ethyl methacrylate and ethyl diethanolamine.
Prior art light-cured coatings had, however, limitations with respect to their scope of applications, especially in uses requiring adhesion to certain polymeric materials. Also, they were generally unsuitable in situations where it was desirable, or mandatory, that the coating could be removed by chemical means, with common non-toxic solvents. It was generally known that monounsaturated acrylate monomers, especially those designed for use in contact with the human body, are difficult or impossible to cure by light using common polymerization-initiating systems, except in blends containing high percentage of di- or polyunsaturated monomers. However, the presence of monounsaturated acrylates at high concentrations is critical in situations when such coatings are intended to be used over, and to adhere to polymers such as acrylics, polystyrene, and polycarbonates. Polyunsaturated acrylic and methacrylic resins have poor characteristics as solvents and, therefore, are unable to effectively bond to the surfaces of polymers, which remain unaffected when exposed to such resins. However, lower (C.sub.1 -C.sub.4) alkyl acrylates and methacrylates possess the ability to soften the surfaces of such polymers, which represents a necessary requirement to allow for effective bonding to them. Consequently, most commercially known acrylate coatings, particularly those used over acrylate polymers in dentistry, are based on such monomers.
Low reactivity of such monomers when cured by light is the reason why they were employed, almost exclusively, in self-cured systems. In light cured formulations of the prior art which contain monounsaturated acrylates, particularly those containing lower alkyl methacrylates, such monomers are present in blends containing, as major ingredients, acrylate monomers having, two or more unsaturated groups per molecule. Their presence was considered necessary to provide an adequate level of reactivity to such blends and, therefore, to assure a desirably short curing time.
It was generally recognized by those skilled in the art that in formulations developed for dental purposes, the polyunsaturated acrylates must represent more than 50% of the blend to achieve an acceptable curing time when using light-emitting dental curing instruments. However, such a high concentration of polyunsaturated monomers impairs the ability of the coating to bond to plastic surfaces and causes excessive brittleness of the coating, resulting from a high cross-linking density in the polymer. Also, a high concentration of polyunsaturated monomers causes a tendency toward crazing and the formation of an undesirable unpolymerized oxygen-inhibited layer on the surface of the cured material. The result is a surface lacking in smoothness, requiring time-consuming cleaning and finishing after cure, and susceptible to damage when impacted by hard objects.